Contact adhesives

ABSTRACT

Disclosed are contact adhesives, containing aqueous polymeric latexes, particularly acrylic latexes. Contact adhesives are adhesives which dry to a non-tacky state but which will adhere to another surface of the same dried adhesive. A unique feature of the adhesives is that the polymer used therein has an extremely wide range of molecular weights with substantial proportions of the product having a low molecular weight and substantial portions having a very high molecular weight, preferably with fairly uniform progression from low molecular weight to high molecular weight. However, blends of a low molecular weight fraction and a high molecular weight polymer may also be used. The preferred procedure is to prepare an emulsion polymer by reacting part of the material in the reactor in such a way as to obtain a high molecular weight product and to react at least one other portion of the monomer in the same reactor in such a way as to obtain at least one low molecular weight fraction. The adhesives preferably contain an aminoplast or a phenoplast as a tackifier and crosslinker and may contain a thickening agent such as polyvinyl alcohol. Also disclosed are articles of manufacture laminated by means of the adhesives and methods of preparing the same.

This is a continuation of application Ser. No. 119,916, filed Feb. 8, 1980, abandoned.

The present invention is concerned with the production of adhesive compositions of aqueous character adapted to be applied as a contact cement. By contact cement is meant a cement which has the capacity to bond two substrates well enough on initial assembly to hold the assembly together without employing extended nip-roll or clamp pressure. The present invention is also concerned with methods of preparing articles of manufacture employing the adhesive compositions of the present invention, and methods for their preparation.

The latex polymers of the invention have substantial proportions of high molecular weight fractions and low molecular weight fractions. The divergent molecular weights may be achieved by blending latexes or, preferably, by a novel polymerization procedure in which staged introduction of chain transfer agent is utilized.

BACKGROUND OF THE INVENTION

Contact cements or adhesives are polymer solutions or dispersions which are applied to two surfaces, dried, and the mating surfaces are then pressed together, usually without heat. The dried surfaces are essentially non-tacky and will not adhere to most materials except to another coating of the same adhesive. Care must be used in aligning the two articles or surfaces to be bonded, since the bonding is essentially instantaneous and the articles cannot be moved relative to one another in order to put them into register once contact has been made. This "instant-grab" property of contact adhesives has the important advantage that long periods of aging or oven curing are not needed. Particularly for on-site applications, such as laminating a plastic surface sheet material to a kitchen countertop, the advantages of the use of contact adhesives are particularly valuable.

Pressure sensitive adhesives are known which involve blends of polymers. For example U.S. Pat. No. 3,090,694 in Examples 7 and 8, disclose blends of acrylic latexes. In Example 7 the first product ("Rhoplex"®FRN latex) is known to have a number average molecular weight (M_(n)) of about 500,000 and the second product ("Rhoplex" B-60A latex) to have a molecular weight of about 1,000,000. In Example 8 the first product ("Rhoplex" B-15 latex) is understood to have a M_(n) in the neighborhood of 750,000 and again the second product ("Rhoplex" B-60A latex) has a M_(n) of about 1,000,000. The glass transition temperature of Rhoplex FRN is about -15° C., that of Rhoplex B-60A is about 13° C. and that of Rhoplex B-15 is about -12° C. Additionally, U.S. Pat. Nos. 3,222,419 and 3,257,478 disclose blends of crosslinking acrylic resins and non-crosslinking acrylic resins, in the form of organic solvent solutions, used as pressure sensitive adhesives.

As is shown by U.S. Pat. Nos. 2,976,203 and 2,976,204 to Young (assigned to the assignee of the present application) it is known to utilize chain transfer agents to lower the molecular weight of acrylic latex polymers useful as pressure sensitive adhesives and as contact cements. Another reference showing that chain transfer agents are known for making pressure sensitive adhesives is U.S. Pat. No. 3,806,484, in which the acrylic monomers are pre-emulsified along with the chain transfer agent, or, as understood, the emulsion is polymerized in increments wherein the chain transfer agent is added in the same quantity to each stage or increment of the emulsion being polymerized.

Formulations for contact cements utilizing acrylic latexes and phenolic resins are discussed in the 1975 edition, Vol. 9A, of Polymer Science and Technology on pages 233-48 (abstracted at Chemical Abstracts Vol. 84 Abstract 136482a). A review of contact adhesives is mentioned in Volume 81 of Chemical Abstracts, Abstract No. 121991e. German patent publication No. 2,420,683, cited in volume 82 of Chemical Abstracts, Abstract No. 59158d, discloses a contact adhesive and refers to U.S. Ser. No. 355,676. Elastomers are known to have a wide range of molecular weight polymer chains.

A patent relating to synthetic rubber, U.S. Pat. No. 4,145,494, shows the polymerization of diolefins, optionally with minor proportions of monoethylenically unsaturated monomers such as acrylic acid, methacrylic acid, itaconic acid, esters thereof, styrene, etc., during which larger amounts of chain transfer agents are added after at least 75% of the monomers are polymerized. A shortstopping agent is added to prevent complete conversion of the monomer to polymer. No such agent is used in the present invention.

One way of carrying out a synthesis procedure to obtain a latex polymer having a wide divergence of molecular weights is taught by U.S. Pat. No. 4,039,500 to Bassett et al. It teaches, in one variation, feeding a stream of unemulsified monomer mixture containing a chain transfer agent into a second body of unemulsified monomer simultaneously with continuous feeding of a stream of the second body of monomer into a polymerization reactor which contains water, an emulsifying agent and an initiator. A polyunsaturated crosslinking monomer is preferably included. Example 1 of the patent is typical, yielding an overall polymer composition of 45/45/5/5 styrene, ethyl acrylate, methacrylic acid, 2-hydroxyethyl acrylate, which has a calculated Tg, defined hereinafter, of about 28° C. The first feed would yield a polymer having a lower Tg and the second feed would yield a polymer having a higher Tg, probably in a so-called shell-core relationship.

That process has deficiencies as compared with the process of Graham Swift and Richard E. Baus, discussed below and disclosed in certain of the examples in the present case. For instance, the Bassett et al. process is difficult to control. The Swift et al. process, on the other hand, allows better control of the proportions of very high molecular weight and very low molecular weight materials. Furthermore, Bassett et al. are concerned with getting a hard polymer for paints, using substantial proportions of monomers such as styrene and methyl methacrylate, which give products having a high Tg. Bassett et al. also recommend using substantial amounts of the unsaturated crosslinker monomers, which give extremely high molecular weight products as well as a high Tg.

Contact adhesives differ from pressure sensitive adhesives in that they are essentially non-tacky (although they adhere to one another), while pressure sensitive adhesives retain a permanent, aggressive tack. This may be illustrated by comparing tack ratings of contact and pressure sensitive adhesives in conventional tack tests such as the Rolling Ball Tack Test. In this test, a 7/16" steel ball is rolled down a 6 inch, 45° inclined steel sheet onto the horizontal adhesive surface in the form of a dry coating of about 1 mil in thickness. A pressure sensitive adhesive is expected to stop the ball after it has travelled only one or two inches across the adhesive surface. Conversely, contact adhesive will permit the steel balls to roll almost indefinitely.

Traditionally, contact adhesives have been based on solutions of poly(chloroprene) or neoprene, in combinations of solvents such as toluene and methyl ethyl ketone. Recent government regulations restricting use of such solvents has sparked adhesive manufacturers to seek alternate systems which pose significantly lower flammability and pollution hazards.

Aqueous adhesives seem to be ideal candidate replacements for the hazardous solvent systems. However, early aqueous neoprene latices lacked the high bond strengths shown by their solvent counterparts, nor are they stable to freezing and thawing. In a subsequent innovation, it was found that the deficiency could be overcome by using specialty acrylic latices such as 87.5 ethyl acrylate/10 methyl methacrylate/2.5 itaconic acid+poly(vinyl alcohol) thickener and benzoguanamineformaldehyde crosslinker. While this may be used without safety hazards to provide high strength wood-to-plastic laminates, the system requires higher lamination pressure than customers desire, and thus greater combinability was sought. Improved combinability, as evidenced by good adhesion with low lamination pressure, can be achieved by incorporation of tackifiers and plasticizing solvents, reducing Tg, or dropping molecular weight. However, in all cases combinability is achieved along with an unacceptable loss is elevated temperature performance.

Extensive testing of contact adhesives has indicated that performance in terms of adhesive properties such as lap shear, high temperature cleavage, legginess, combinability, etc., is related to the type and amount of acid functionality in the polymer backbone and the type and amount of crosslinker. However, it is apparent that performance is even more dramatically related to molecular weight of the polymer. High molecular weight improves lap shear adhesion and cleavage properties, but downgrades legginess, combinability, and bond fusion. The exact opposite is true of low molecular weight polymers.

DETAILED DESCRIPTION

This invention provides contact adhesives containing polymers or polymer fractions with a range of molecular weights from very high to very low and of the proper balance to give good adhesive strength in conjunction with good legginess, combinability and bond fusion. To achieve this, in a preferred embodiment of the invention a substantial portion of the monomers are polymerized to a high molecular weight. The high molecular weight polymer provides adhesive strength and high temperature cleavage properties. Another portion of the monomers is polymerized to a low molecular weight fraction, also present in a substantial proportion, which imparts legginess and combinability.

Typical molecular weight distributions, determined by gel permeation chromatography, are shown in the drawings. FIG. 1 concerns the product of Ex. G, FIG. 2 concerns the product of Ex. N, and FIG. 3 represents the product of Ex. J-1.

The adhesive composition of the present invention are aqueous dispersions of a water-insoluble copolymer or mixed copolymers, preferably acrylic in nature, and water insoluble salts thereof, of 0.5-15%, preferably 1-12% by weight of a carboxylic acid selected from the group consisting of acrylic acid, itaconic acid, maleic acid, fumaric acid, fumaric acid, methacrylic acid, and crotonic acid, and mixtures thereof, more preferably a mixture 5-10% of methacrylic acid with 0.5-2% itaconic acid, 0 to 25% by weight of another addition-polymerizable α,β-monoethylenically unsaturated monomer, including one or more of an ester of methacrylic acid with an alcohol having 1 to 18 carbon atoms such as methyl methacrylate, and/or other addition polymerizable monomers such as vinyl versatate, styrene, vinyl toluene, vinyl acetate, or acrylonitrile etc., or mixtures thereof, and the balance of one or more esters of acrylic acid with an alcohol having from 1 to 18 carbon atoms, preferably 1-8 carbon atoms. The most preferred compositions consist essentially of polymerized ethyl acrylate, butyl acrylate and the unsaturated acid or acids.

The acid component is needed to provide specific adhesion to a wide variety of substrates and especially to those of hydrophilic or metallic character, and as a catalyst to cause curing of an aminoplast component, described hereinafter, when used.

The particular ester of acrylic acid that is employed may be chosen from a wide group including methyl acrylate, ethyl acrylate, butyl acrylate, cyclohexyl acrylate, 2-ethylhexyl acrylate, t-octyl acrylate, dodecyl acrylate, and octadecyl acrylate. Preferably, the copolymers are made using methyl acrylate, ethyl acrylate, butyl acrylate, or 2-ethylhexyl acrylate, because of the availability, efficiency and/or inexpensiveness of those monomers.

Optional crosslinking monomers such as vinyl benzene, diallyl phthalate may be included in small amounts of 0-2%, preferably 0-0.5% of the monomers.

Acrylic copolymers are preferred, but the invention may also be applied to the preparation of other polymers of addition polymerized unsaturated monomers at least predominantly composed of α,β-monoethylenically unsaturated monomers. Examples are polyvinyl acetate and a copolymer of ethylene and vinyl acetate, optionally with small amounts of other monomers such as hydroxy ethyl, hydroxypropylacrylate and methacrylic N-methylol acrylamide, acrylamide, acrylic acid, methacrylic acid, or itaconic acid, and up to 20% of another optional copolymerizable monomer. Suitable ratios of ethylene to vinyl acetate are 30 to 70 parts of ethylene to 30 to 70 parts of vinyl acetate by weight. The chain terminator may be added similarly to the examples in the present application, in which the vinyl acetate is emulsified and polymerized by pressuring with ethylene during the first portion of the polymerization to give a higher molecular weight product, followed by additional increments of vinyl acetate and ethylene in the presence of chain regulators. In a continuous process, the initial stage or stages of polymerization are conducted in the absence of chain regulator which is then introduced downstream in one or more subsequent sections of the reactor.

The composition may have a pH of about 2 to about 7 or higher but preferably the pH is between 5 and 7. Proportions of the several components of the copolymer are such as to provide a T_(g) between 10° C. and -60° C., preferably from -5° C. to -40° C.

"T_(g) " or glass transition temperature, is described by Flory, "Principles of Polymer Chemistry," pp. 56 and 57 (1953), Cornell University Press. See also "Polymer Handbook", 2nd Ed., Brandrup and Immergut, Sec. III, pp. 139-142, Interscience (1975). While actual measurement of the T_(g) can be used, it is difficult to obtain an accurate value on low molecular weight polymers, and it may be calculated as described by Fox, Bull. Am. Physics Soc. 1,3, p. 123 (1956), or by the use of "Rohm and Haas Acrylic Glass Temperature Analyzer" Publication No. CM-24 L/cb, Rohm and Haas Company, Philadelphia, Pa., 19105, which are incorporated herein by reference. The actual T_(g) 's of the low molecular weight polymers are lower than the calculated T_(g) because of low molecular weights, e.g., note Example B, which product has a calculated T_(g) of 22° C., but it is actually a viscous liquid at that temperature. The calculated T_(g) values, which are essentially the same as measured T_(g) 's of high molecular weight (>100,000, M_(n)) polymers, are relevant indicia of the relative T_(g) 's of different polymers.

The T_(g) value referred to is the transition temperature or inflection temperature which is found by plotting the modulus of rigidity against temperature, determined at 300 kg./cm.².

An essential characteristic of the copolymer is that it be of a very broad molecular weight range. The range of molecular weights is obtained by the employment of a chain regulator in an emulsion polymerization procedure as described in more detail hereinafter, or by blending polymers of widely varying molecular weights. The amount of chain regulator may vary, depending upon the particular regulator and the particular conditions of polymerization, but in general from 0.05 to 5% and preferably 0.2 to 1.5% by weight of such chain regulator is employed, such percentage being based on the weight of monomers being polymerized in the presence of free chain regulator.

The polymer compositions must have an extremely wide range of molecular weights. This may be achieved by blending polymers having extremely low molecular weights with those having high molecular weights, or preferably by the method taught in copending U.S. patent application Ser. No. 119,728 filed on an even date herewith in the names of Graham Swift and Richard E. Baus, titled "Emulsion Polymer of Heterogeneous Molecular Weight and Preparation Thereof," now abandoned, and in its continuation, Ser. No. 313,400, filed Oct. 22, 1981. A convenient way of expressing the wide divergence of molecular weights is by means of the "heterogeneity index". This is the ratio of the weight average molecular weight (M_(w)) to the number average weight (M_(n)). In accordance with the present invention the heterogeneity index is between 15 and 150, preferably 20 to 90 based on molecular weights determined by gel permeation chromatography.

Although the heterogeneity index gives an indication of the range of molecular weights of the polymer fractions in the material, it does not tell the whole story. Thus the gel permeation chromatograms (GPC's) indicate relative quantities of low molecular weight and high molecular weight products. For example, in a conventional emulsion polymerization conducted in the absence of the chain transfer agent, followed after polymerization by utilization of an additional quantity of catalyst or initiator in order to chase residual monomer, the polymer may have a wide molecular weight range of polymer fractions, but the bulk of the polymer, commonly more than 95% of it, is of extremely high molecular weight, well above a million, and only a minor fraction has an extremely low molecular weight. In view of this, in order to further define the invention, another parameter is the relative ratios of low molecular to high molecular weight products and limitations on those molecular weights. Thus, the invention also requires that between about 5% and 70% by weight, preferably between about 10% and 60%, of the product is a low molecular weight fraction having an M.sub. n of between about 500 and about 100,000, preferably from about 10,000 to 50,000 and between about 30% and 95%, preferably between about 40% and 90%, of a high molecular weight fraction having a M_(n) of between about 100,000 and about 2,000,000 preferably from about 500,000 to 1,500,000. It is to be understood that ordinarily, as t the preferred M_(n) fractions, additional fractions of intermediate molecular weight polymer may be present.

Although, as indicated elsewhere herein, the preferred procedure for obtaining products having a high heterogeneity index, as calculated from the ratio of the M_(w) /M_(n), is by initially polymerizing an emulsion of monomers in the absence of a chain terminator, and part way through the polymerization introducing a chain transfer agent in one or more increments and quantities, an alternative procedure for obtaining suitable products having a wide range of molecular weights is to blend an extremely low molecular weight product with a high molecular weight emulsion polymer. The low molecular weight products may be obtained by various procedures. They may be post-emulsified bulk polymers or solution polymers, as well as low molecular weight emulsion polymers. The low molecular weight products, suitably called oligomers, can be prepared by anionic initiation, using a catalyst such as potassium methoxide, as disclosed in U.S. Pat. No. 4,103,093, and British Pat. No. 1,431,446 (corresponding in large part to U.S. Ser. No. 629,186, filed Nov. 5, 1975, now U.S. Pat. No. 4,158,736). Other procedures are useful for preparing the low molecular component(s) of the blend. Examples appear in the journal article by Ali, Mark, & Mesrobian, Ind. Eng. Chem. 42, 484 (1950); the article by Leonard, Szlachtun, and Cort, J. Poly. Sci., XI, No. 6, 539 (1953); Rehberg & Siciliano, Ind. Eng. Chem., 44, 2864 (1962) and Bauer, Chimicha, 5, 147 (1951). A number of these low molecular weight polymers have been disclosed as components of blends with polyvinyl chloride to plasticize the same, in the form of alkali soluble salts of high unsaturated acid level polymers to modify acrylic latexes such as floor polishes which must provide a hard, wear-resisting surface, and the like. In the present case, the low molecular weight polymer fraction should also have a low T_(g) in the same range as given above, that is, from -60° C. to 10° C., preferably less than -5° C. and greater than -40° C.

In the place of blends, it has been found that a highly desirable balance of combinability and elevated temperature performance can be achieved through use of a graduated addition of chain transfer agent (CTA) during formation of an emulsion copolymer, particularly acrylic polymers, prepared in accordance with the teaching of Swift and Baus, supra. In this way, a variety of molecular weight species is formed, and the desired combination of properties is achieved. Thus, a part of the polymerization is conducted in the absence of a free CTA, and in at least one stage of the polymerization, preferably in two stages, the polymerization is conducted in the presence of free chain transfer agent.

If the CTA is added in a first stage, it is used in a quantity such that it is used up by being bound in polymer chains, thus permitting adding additional monomer in the absence of free CTA.

Preferably the first stage is free of CTA. Using this method, there exists a rather narrow optimum range for introduction of CTA. To provide the optimum property balance, CTA addition preferably is not begun before at least 30%, preferably at least 45% of the monomer mix has reacted to high molecular weight product. However, the chain transfer agent addition is commenced before 95%, preferably 90%, and more preferably 75%, of the monomer mix has reacted to completion. Thus, preferably between 45% and 75% of the acrylic monomer is reacted to polymer prior to gradual incorporation of the initial chain transfer agent charge along with monomer emulsion. A second higher amount of CTA is preferably introduced with at least one later portion of monomer.

The water insoluble salt of the copolymer may be that obtained by the use of ammonia, an alkali metal, such as sodium or potassium, or a water-soluble amine such as a lower aliphatic amine of which triethylamine, diethylamine, and trimethylamine, dimethylethanolamine, diethanolamine, or triethanolamine. Preferably ammonia or a volatile amine is employed as the cation of the salt of the acid copolymer.

Examples of anionic emulsifying agents that may be used for emulsifying the monomers include the higher fatty alcohol sulfates, such as sodium lauryl sulfate, the alkylaryl sulfonates, such as the sodium salt of t-octylphenyl sulfonate, the sodium dioctyl sulfosuccinates and so on. Examples of the nonionic dispersing agents that may be used for preparing the monomeric emulsions before copolymerization or dispersions of the polymer after polymerization include the following: alkylphenoxypolyethoxyethanols having alkyl groups of about 7 to 18 carbon atoms and 10 to 60 or more oxyethylene units, such as heptylphenoxypolyethoxyethanols, octylphenoxypolyethoxyethanols, methyloctylphenoxypolyethoxyethanols, nonylphenoxypolyethoxyethanols, dodecylphenoxypolyethoxyethanols, and the like; polyethoxyethanol derivatives of methylene linked alkyl phenols; sulfur-containing agents such as those made by condensing 6 or 60 or more moles of ethylene oxide with nonyl, dodecyl, tetradecyl, t-dodecyl, and the like mercaptans or with alkylthiophenols having alkyl groups of 6 to 15 carbon atoms; ethylene oxide derivatives of long-chained carboxylic acids, such as lauric, myristic, palmitic, oleic, and the like or mixtures of acids such as found in tall oil containing 10 to 60 oxyethylene units per molecule; analogous ethylene oxide condensates of long-chained alcohols, such as octyl, decyl, lauryl, or cetyl alcohols, ethylene oxide derivatives of etherified or esterified polyhydroxy compounds having a hydrophobic hydrocarbon chain, such as sorbitan monostearate containing 10 to 60 oxyethylene units, etc.; block copolymers of ethylene oxide and propylene oxide comprising a hydrophobic propylene oxide section combined with one or more hydrophilic ethylene oxide sections.

For the copolymerization, peroxidic free-radical catalysts or initiators, including catalytic systems of the redox type, are recommended. Such redox systems, as is well known, are combinations of oxidizing agents and reducing agents such as a combination of potassium persulfate and sodium metabisulfite. Other suitable peroxidic agents include the "per-salts" such as the alkali metal and ammonium persulfates and perborates, hydrogen peroxide, organic hydroperoxides such as tert-butyl hydroperoxide and cumene hydroperoxide, and esters such as tert-butyl perbenzoate. Other reducing agents include water-soluble thiosulfates and hydrosulfites. Activators or promoters in the form of the salts such as the sulfates or chlorides of metals which are capable of existing in more than one valence state such as cobalt, iron, nickel, and copper may be used in small amounts.

In the polymerization process the greater the proportion of chain regulator (CTA) used the lower the molecular weight obtained. Examples of well know chain regulators or chain transfer agents which may be used include short and long-chain alkyl mercaptans, e.g., amylmercaptan, n-dodecylmercaptan t-dodecyl mercaptan, dialkyl xanthogen disulfides, e.g., diisopropyl xanthogen disulfide, mercaptocarboxylic acids such as mercaptopropionic acid, alkaryl mercaptans such as benzyl mercaptan, long-chain alcohols, such as lauryl alcohol and t-octyl alcohol, halogenated hydrocarbons, such as CCl₄ and CBrCl₃, and substituted mercaptans such as hydroxylethyl mercaptan.

The copolymer dispersions may be made by first mixing the several monomers in the desired proportions into an aqueous solution of an anionic or a non-ionic dispersing or emulsifying agent, or a mixture of both types, to form an emulsion, then polymerizing the mixture, with the aid of a free radical catalyst or initiator. The amount of catalyst can vary but for purposes of efficiency from 0.01 to 1.0%, based on the weight of the monomers, of the peroxidic agent and the same or lower proportions of the reducing agent, if used, are recommended. In this way it is possible to prepare dispersions which contain as little as 1% and as much as 60% or 70% of the resinous copolymer on a weight basis. It is, however, more practical--hence preferred--to produce dispersions which contain about 30 to 60% resin solids.

As is conventional, after the monomer feed is completed, residual monomer is reduced to a minimum by use of a free radical "chaser" catalyst or initiator, by the use of an adsorbent, by heat, by means of a vacuum or a combination thereof.

After the polymerization, in the event the unsaturated acid is employed therein as a free acid, neutralization of the dispersed copolymer may be obtained, if desired, by the addition of concentrated ammonia, one of the amines mentioned hereinabove, or the hydroxide or carbonate of sodium, potassium, or lithium. If desired, further adjustment of the concentration may be effected by diluting the neutralized dispersion with water. When a filler is employed, the adjustment of the concentration may be effected after the incorporation of the filler instead of before if desired.

The unmodified or unfilled composition may contain 30 to 55% water and at least 30% of the copolymer or salt thereof. For optimum results, however, the composition comprises about 50 to 60% water, about 40 to 55% of the copolymer, copolymer mixture, or salt thereof, and small amounts, up to about 5% total of the auxiliary materials, such as the emulsifier, crosslinker, thickener, and so on.

In the composition of the present invention, a filler may be employed in an amount ranging from 0% up to 125%, preferably by weight of the polymer in the aqueous dispersion. As is known in the art, the filler increases the tensile strength of the adhesive film obtained from the polymer and it reduces the cold flow or creep thereof. This effect, which may be referred to as a reinforcing effect, is definitely noticeable even when as small an amount as 12% of filler, based on the weight of the polymer, is employed. The maximum reinforcing effect is obtained when the filler is used in greater amounts up to 125% of the polymer weight, the maximum depending upon the particular filler employed.

The fillers preferably have a particle size not over 40 microns and in optimum cases their dimensions are 10 microns or less. Many types of fillers may be employed. One is rutile titanium dioxide. Others such as anatase titanium dioxide, so-called fused silica, lithopone, magnesium silicate, clay, wollastinite, zinc oxide, calcium carbonate, clays including kaolin and bentonite, walnut shell, and so on, are useful. To some extent fibrous fillers such as cellulosic fibers or nylon staple may be employed. Inorganic fillers are preferred.

There may be added to the adhesive composition, to improve the viscosity and flow properties thereof, from 0%, preferably 0.25% to 10% by weight, based on the total solids of the composition, of a thickening agent. The amount of such thickening agent, when used, is generally selected to provide a viscosity in the composition of about 1/2 poise to about 100 poises. In most cases, when a viscosity in the lower part of the range is desired in the particular application, no thickener is necessary. Examples of thickening agents include natural gums, such as gum tragacanth and gum arabic, polyvinyl alcohol, hydrolyzed methacrylic acid and water soluble salts, thereof, and water-soluble cellulose ethers, such as methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, and the like.

The adhesive usually contains a crosslinking means such as crosslinkable aminoplast, e.g., an alkylated one. The alkylated aminoplasts which may be used include those obtained by the alkylation, with an alkanol having from 1 to 6 carbon atoms or cyclohexanol, of a condensate of an aldehyde with urea, N,N'-ethyleneurea, dicyandiamide, and aminotriazines. Water-soluble condensates such as the methylated dimethylolurea condensates can be employed. Preferably, the alkylation products of alcohols having from 3 to 6 carbon atoms are employed and the butylated products are particularly valuable.

Among the aminotriazines which are suitable are melamine, acetoquanamine, benzoguanamine, formoguanamine, N-(t-butyl)-melamine, N-(t-octyl)-melamine in which the t-octyl group has the formula

    --C(CH.sub.3).sub.2 --CH.sub.2 --C(CH.sub.3).sub.3

ammeline, 2-chloro-4,6-diamino-1,3,5-triazine, 2-phenylpoxy-4,6-diamino-1,3,5-triazine, 6-methyl-2,4-diamino-1,3,5-triazine, 2,4,6-trihydrazine-1,3,5-triazine, 2,4,6-triethyltriamino-1,3,5-triazine, and the N,N-di-(C₁ -C₄)alkyl melamines such as N,N-dimethylmelamine. While any aldehyde may be employed such as acetaldehyde, crotonaldehyde, and acrolein, the condensates obtained using formaldehyde and revertible polymers thereof such as paraformaldehyde and preferably employed.

In place of the aminoplast crosslinking system other known latent crosslinking means or mechanisms may be used. For example, phenoplasts such as the water soluble phenol formaldehyde resins referred to in the Azrak et al. journal article, supra, or in German Pat. No. 2,420,683 may be utilized. Crosslinkable monomers such as N-methylol acrylamide are also of use. Another well known mechanism is to utilize ionic crosslinking by polyvalent metal ions. An example is zinc ammonium carbonate or other polyvalent metal compounds or complexes having a volatile ligand. Still another known procedure is to include an organic peroxide in the adhesive formulation at the point of use which subsequently interacts with the acrylic polymer to cause crosslinking. These latent crosslinking reactions tend to minimize cold flow of the adhesive subsequent to its utilization. Of course, other crosslinking mechanisms and procedures may be utilized, such as the use of polyunsaturated monomers. Examples of such optional cross-linking agents that may be used, in amounts of 0-2%, preferably 0-0.5%, include any copolymerizable compound which contains two or more non-conjugated points of ethylenic unsaturation or two or more non-conjugated vinylidene groups of the structure, CH₂ ═C<, such as divinyltoluene, divinylbenzene, trivinylbenzene, divinylnaphthalene, ethylene glycol diacrylate or dimethacrylate, trimethylene glycol diacrylate or dimethacrylate, 1,4-butylene glycol diacrylate or dimethacrylate, 2-ethylhexane-1,3-dimethacrylate, divinylxylene, divinylethylbenzene, divinyl ether, divinyl sulfone, allyl ethers of polyhydric compounds such as glycerol, pentaerythritol, sorbitol, sucrose and resorcinol, divinylketone, divinylsulfide, allyl acrylate, diallyl maleate, diallyl fumarate, diallyl phthalate, diallyl succinate, diallyl carbonate, diallyl malonate, diallyl oxalate, diallyl adipate, diallyl sebacate, diallyl tartrate, diallyl silicate, triallyl tricarballylate, triallyl aconitate, triallyl citrate, triallyl phosphate, N,N'-methylenediacrylamide, N,N'-methylenedimethacrylamide, and N,N'-ethylidenediacrylamide.

Generally, it is unnecessary to add any material other than the acid groups present in the emulsion polymer to catalyze the aminoplast crosslinking reaction but, if desired, an acidic catalyst may be included. The amount of such a catalyst may be from 0.1 to 1% by weight, based on the weight of aminoplast condensate. The use of the curing catalyst may be particularly desirable when lower temperatures of curing or baking are needed. With such curing catalysts, insolubilization can be accomplished simply by drying and aging at room temperature. Among the curing catalysts that may be used are any of the organic and inorganic acid catalysts. One may use, for instance, in catalytic amounts, sulfuric acid, hydrochloric acid and their acid salts, such as ammonium sulfate, ammonium chloride, or the organic acids, such as acetic acid, phthalic acid, benzoic acid, toluene sulphonic acid, naphthalene sulphonic acid, and the mono-salt of maleic acid with triethylamine. Such catalysts are added at the point of use. "Self-contained" acid catalysts provided by the unsaturated acid in the polymer are preferred. Itaconic acid is especially valuable in that, not only is it a stronger acid than acrylic or methacrylic acid, thus accelerating the aminoplast cure, but it also gives a stronger adhesive bond. The combination of itaconic acid and methacrylic acid in the emulsion polymer is of particular value.

The adhesive compositions may be applied in various ways. For example, they may be applied by brush, roller, trowel, knife, dipping, spraying, and so on. The adhesive is applied to both surfaces to be adhered together and then the coatings are dried. When both surfaces are porous, the cement may be applied to both and then contacted very quickly thereafter.

The strength of bond develops instantaneously to hold the substrates together. As the filler content increases, the bond strength develops more rapidly. If the amount of filler is such as to allow only five minutes open-assembly time, greater strength is obtained in one hour after contact then if lower filler content is used.

The adhesive of the present invention has been found to provide exceptionally good bonds with numerous substrates, including structural materials such as wood, paper, Formica® branch plastic sheet, other plastic materials such as polymethyl methacrylate, polyvinyl chloride, saran, nylon, phenol-formaldehyde resins, urea-formaldehyde resins, and other thermoset materials such as melamine-formaldehyde resins; rayon, cotton, silk, wool, fibers of the polymeric materials mentioned above, leather, linoleum, rubber, asphalt tile, vinyl tile, ceramic tile, various silicates such as mineral wool, asbestos, concrete, brick, asbestos cement, plaster, metals such as aluminum steel, iron, copper, zinc, chromium, nickel, as well as painted or enamelled surfaces, such as painted automobile bodies, woodwork, walls, ceilings, or floors.

The adhesive composition may be colored or substantially colorless. Besides depending on the color of fillers mentioned above, suitable colored pigments may be added in place of, or in addition to, the fillers. Also, if desired, direct dyes may be introduced to provide a desired color.

The molecular weights in the examples were determined by gel permeation chromatography. Gel permeation chromatograms are run on equipment commercially marketed by Waters Assoc. of Marlboro, Mass. Styragel™ columns are available prepacked in a variety of porosities. A column set is normally composed of four or five four foot sections chosen to cover the molecular weight range to be measured. With column sets of this length sufficient resolution is obtained so that axial dispersion can be disregarded. The column set must be calibrated for the polymer type whose molecular weight is to be determined. Narrow molecular weight standards are available for polystyrene. Calibration curves for other polymers such as polymethyl methacrylate are constructed from the chromatograms of broad samples using either an approximate distribution technique¹ or a universal calibration curve technique². To calculate the molecular weights of unknown samples a table is prepared of the value W_(i), of the GPC curve above baseline at equal volume increments and the molecular weight M_(i), read from the calibration curve at these volumes. The weight average molecular weight M_(w), and the number average molecular weight M_(n), can be calculated from these values with the equations ##EQU1## It is to be realized that the molecular weight numbers given are approximate and not precise, but are validly used for comparative purposes.

TEST METHODS A. Early Grab Testing 1. Measurements Using Heavy Finger Pressure

A moderately heavy brush (5-15 mils when wet) coat of contact adhesive is applied to a 1"×3"×1/20" Formica® brand melamine-formaldehyde plastic strip, as well as to a 3"×6"×1/2" particle board (40 lb. density). When both surfaces are dry (at clear point), the Formica plastic strip is placed on the particle board and laminated using heavy thumb pressure by rubbing the thumb (with pressure) 6× along each edge of the Formica plastic strip (actual bond area is 1"×2"). The laminate is then qualitatively rated for strength, legs, fusion and bond area, immediately after bonding.

2. Measurements Using Specified Weights

A moderately heavy brush coat (5-15 mils when wet) of contact adhesive is applied to a 2"×4"×1/20" Formica plastic strip and a 3"×6"×1/2" wood particle board. At clear point, the Formica plastic strip is laid across the particle board and the Formica plastic strip surface is very lightly rubbed to assure some adhesive contact area. The sample is then lifted by the overhanging edges (actual bond area is 2"×3") of the Formica plastic strip and suspended for 15 seconds with just the weight of the particle board. If the adhesive bond holds for 15 seconds, a qualitative rating of strength, legs and fusion is determined. The early grab test is then repeated, after roller pressure using just the weight from 2 passes of a 4.5 lb. rubber roller (7 psi). The entire process is then repeated using 2 passes of a 20 lb. roller (2 psi).

B. Lap Shear Adhesion

A thin coat (1-3 mils, dry) of adhesive is brush applied to plastic laminate (1"×3"×1/20" Formica sheet) and to particle board (3"×6"×1/2"). After drying until clear (30-45 minutes), another thin coat (1-3 mils, dry) of adhesive is applied to both previously coated surfaces and again allowed to dry until clear. Within a few minutes after its clear point, the two substrates are bonded together, first with heavy finger pressure and then with two heavy hand pressure passes of a small roller over the assembly. The actual bond area is 1 square inch with a 2 inch Formica plastic strip "handle". The samples are conditioned for 4 days at 77° F. and 50% relative humidity (RH), and then pulled apart by a shear stress, using an Instron Tester at a crosshead speed of 0.2"/min.

C. 158° F. Static Cleavage

Samples are prepared in essentially the same manner as lap shear samples, but with the following differences:

1. The wooden substrate is changed to 3"×6"×1/4" Grade A plywood;

2. Laminate area is 1"×2" long, with a 1" Formica plastic strip "handle";

3. Samples are conditioned for 7 days at 77° F. and 50% R.H.

Actual testing is done by placing the laminates in a 158° F. circulating air oven with a 1 kg weight hanging vertically from the outer edge of the 1" Formica plastic strip "handle". End point is noted at complete failure.

D. Legginess

A criterion sometimes used in the industry for preliminary evaluation of a contact adhesive is the extent to which "legginess" appears in the adhesive upon delamination of two surfaces adhered together. While it is believed that the data herein show that legginess is not essential to a good contact adhesive, it is a quality desired by some users. By legginess is meant the characteristic of the adhesive, when the laminate is forceably delaminated to remain adhered to both surfaces while separating in the nature of stringy connections between the two surfaces as they are pulled apart. If the legs or strings develop over a large proportion of the area adhered together and retain their string shapes when about 0.1 centimeter or more in length, before breaking by cohesive failure, or before delaminating from one or the other surfaces by adhesive failure, the legginess is rated as very good. Of course it is a subjective rating and if there are no legs found the rating is zero. A rating of P is poor, of F is fair, of G is good, of VG is very good, and of Ex. is excellent. Intermediate ratings are stated as, for example, P-F meaning poor to fair.

E. Freeze-Thaw

The latex is frozen for 16 hours at -15° C. and thawed for 8 hours. If the latex does not coagulate after from one to five such cycles, it is deemed to be freeze-thaw stable.

In the following examples the Gelvatol® 20-30 polyvinyl alcohol has a viscosity in centipoises of a 4% aqueous solution at 20° C. of 4-6, the percent residual polyvinyl acetate determined on a weight basis is 19.5-21.5% the percent hydrolysis of the polyvinyl acetate is 87.7-89%, the weight average molecular weight is approximately 10,000, and the percent ash is 0.75. Gelvatol is a trademark of the Monsanto Company. The Cymel™ is a benzoquanamino or melamine-formaldehyde condensate produced by Cyanamid. The Aerotex® resin M-3 is also a product of Cyanamid and is a melamine-formaldehyde condensate, and is a known crosslinker for contact adhesives and similar products. Suitable phenolic resins are disclosed in an article by Azrak et al. in Adhesives Age, June, 1975, pages 23-28. Phenolic resins are useful in place of the Aerotex product. One such product is designated Bakelite® phenolic resin CKRA-1834. Other such phenolic tackifiers are designated BXKU-6387 and BKUA-2260, BKUA-2370, 45-48% aqueous dispersions, 40-50 parts of dispersion per 100 parts of 45-60% solids latex. The latter two types of products are water insoluble and have a labile formaldehyde content of 5 to 7%, prepared by first dissolving the phenolic resin in a solvent and emulsifying the same. For example, 100 parts of the phenolic resin with 54 parts of toluene and 6 parts of oleic acid is combined with 6 parts of 25% aqueous sodium hydroxide, and 30 parts of 10% ammoniated casein in water, 30 parts of 5% gum arabic in water, and 77 parts of water. The first three ingredients are combined and then introduced into a mixture of the last four ingredients with high speed agitation. This is then mixed with the latex and other conventional contact adhesive ingredients.

The following are the abbreviations utilized in the examples for monomers:

    ______________________________________                                         ABBREVIATION       MONOMER                                                     ______________________________________                                         AA                 acrylic acid                                                BA                 butyl acrylate                                              BDA                1,4-butylene diacrylate                                     EA                 ethyl acrylate                                              EHA                2-ethylhexyl acrylate                                       IA                 itaconic acid                                               IDA                isodecyl acrylate                                           IDMA               isodecyl methacrylate                                       LMA                lauryl methacrylate                                         MAA                methacrylic acid                                            MMA                methyl methacrylate                                         S                  styrene                                                     VV-10              vinyl versatate                                             ______________________________________                                    

In the following examples, which are illustrative of the invention, the parts and percentages are by weight unless otherwise noted.

EXAMPLE A Anionically Initiated Oligomer of BA/AA, 77.5/22.5

The end product is oligomeric butyl acrylate/acrylic acid in the approximate weight ratio of 65/35. It is prepared in two stages. In the first stage, oligomeric butyl acrylate is prepared, which, in a second stage, is saponified with potassium hydroxide to the final product. It has a very narrow heterogeneity index of less than 3.

1. Preparation of Anionically Initiated Oligomeric Polybutyl Acrylate

    ______________________________________                                                          Lbs./100 Lbs. Product                                         Raw Materials    (Prior to Strip and Filtration)                               ______________________________________                                         Butyl Acrylate (BA; 5 ppm                                                                       87.50                                                         MEHQ; 0.05% H.sub.2 O)                                                         Potassium tert-Butoxide                                                                         1.53                                                          (t-BuOK; powder)                                                               Toluene (0.04% H.sub.2 O)                                                                       9.90                                                          Sulfuric Acid (98.5-99.0%)                                                                      0.71       (322 g.)                                           Hyflo Super-Cel  0.44       (199.6 g.)                                         Filter Aid       100.08                                                                       (Stripped, filtered product                                                    recovery ca. 78-80 lbs.)                                        ______________________________________                                    

Butyl acrylate (BA) monomer is added gradually over 1 hour to a stirred slurry of potassium tert-butoxide (2 mole %) in toluene, initially at 25° C., under a dry N₂ atmosphere. Oligomerization occurs upon addition and the resultant exotherm is allowed to carry the reaction temperature to 70° C. This temperature (70° C.±3°) is maintained throughout the addition, with cooling and heating as needed, and during the subsequent 3 hour hold. During the initial stage of addition, the slurry thickens and turns orange, but within several minutes a nearly clear, orange solution results. Color persists throughout the reaction time. Conversion at the completion of the 1 hour addition is approximately 75%; conversion increases to about 90% during the hold period.

The slightly hazy, orange, mobile reaction mixture is quenched with sulfuric acid, treated with 0.5 wt. % Hyflo Super-Cel and subjected to a vacuum strip (20 mm.) at 70°-75° C. with a slow N₂ sparge for 0.5 hours. The stripped material is filtered warm to give a clear, straw to light yellow oil at 97% solids content, having the following properties:

    ______________________________________                                         Percent Solids   97% (100°/60 min., forced                              (Nonvolatiles)   draft oven)                                                   Viscosity        500 cps/25° C.                                         Appearance       Clear; straw to light yellow                                  Carboxylic acid content                                                                         0.05 meq./g.                                                  Molecular weight M.sub.n (oven-dried, 100°/60 min.)                                      900 ± 30; gpc maximum wt.                                                   fraction ca. 1700-1800.                                       ______________________________________                                    

2. Saponification

Aqueous potassium hydroxide (45 wt. %), at a level of 57 equivalent % of total BA ester, is added over one-half hour to a stirred solution (70% solids) of the product of the first stage in isopropanol, initially at 40° C. Mild exotherm, color formation (nearly colorless to yellow-orange), and clouding of the solution accompany addition, with the exotherm diminishing as addition progresses. Approximate cooling is required to maintain the system at 40° C. (±5°). After addition, conversion is 85-90%, saponification is carried to completion during the subsequent distillation step. Prior to distillation, water is added lowering solids to 30% and providing a clear, amber solution of maximum process volume. This low solids system is needed both to minimize foaming tendency which occurs near the end of the distillation and to solubilize inorganic salts produced by subsequent acidification.

Distillation at atmospheric pressure removes isopropanol and butanol as their water azeotropes. An alcohol-free system is obtained by continuing the distillation until only water (head temp. 100° C.) is removed. The resultant clear, 48% aqueous solution of the carboxylate salt is reduced to 40% solids with additional water and then taken to 60° C. The extraction/isolation solvents (a 1/1 toluene/MIBK mixture; salt and organic solvent are ca. 60% solids; total solids are 30%) are then added. (MIBK is methyl isobutyl ketone.) Acidification of the hazy system follows at 60° C. with the addition of concentrated sulfuric acid (150 equivalent % on KOH charge) via dip tube over a 15 minute period. Exotherm accompanies acidification and the temperature is allowed to rise to 65°-70° C. and is maintained with cooling. After acidification the white mixture is stirred for 15 minutes at 65° C. Agitation is stopped and clean separation of the warm layers occurs in less than 30 minutes.

Separation affords a nearly clear, lower aqueous layer containing a small amount of fine solid and a slightly hazy greenish yellow, upper organic layer. The lower layer containing 10-15 equiv. %/5 wt. % of oligomeric acid (primarily low molecular weight oligomer of high acid content), as well as potassium sulfate and bisulfate, is drawn off and discarded. The MIBK/toluene solution of the crude product (50% solids) containing residual water and inorganic salts, is subjected to vacuum azeotroping and distillation (200-250 mm Hg.; maximum pot temp. 85° C., maximum head temp. 65° C.), increasing solids content to 80-82% Diatomaceous earth filer aid (0.5 wt. %) is thoroughly mixed into the warm, mobile, slightly hazy, yellow oil and filtration through a warm pressure funnel at 30 psi proceeds smoothly (20 lbs./ft.² hr.) to provide a clear yellow syrup of 80-82% solids content and 4.2-4.4 meq. carboxylic acid/g. solution. The product has properties falling within the following limits:

    ______________________________________                                         Appearance:     clear, light yellow                                            % Solids (Nonvolatiles)                                                                        80-82% (methyl isobutyl ketone/                                                toluene; 125° C./60 min., forced                                        draft oven, 1.0 g.)                                            Viscosity       6500-7000 cps/25° C.                                    Carboxylic Acid Content                                                                        4.2-4.4 meq./g. solution                                       Acid No. (100% solids)                                                                         290-300                                                        ______________________________________                                    

The process described above applies to 57 eq. % saponification of the oligio-BA backbone (M_(n) ca. 900). However, the process is readily adapted to a saponification range of 42 to 57 eq. %, giving products of acid numbers ranging from about 200 to 300. The saponification level chosen requires only minimal changes (i.e. base and acid charges) of the process.

These products are described in U.S. patent application Ser. No. 629,186 filed Nov. 5, 1975 which corresponds in large part to Belgian Pat. No. 794,403 and British Pat. No. 1,431,446. The U.S. application has now matured into patent No.

EXAMPLE B Preparation of Free Radical Initiated Oligomer of Ethyl Acrylate and Acrylic Acid

Reaction is carried out in a five liter, four necked round bottom flask equipped with a paddle stirrer, addition funnel, equipment for nitrogen sparging and a Y adapter fitted with a thermometer and two reflux condensers (in line). Cellosolve acetate (2000.0 g) is charged to the kettle and heated with an oil bath to reflux (156° C.) under a slow nitrogen sparge, the sparge being discontinued once reflux is reached. The initiator and monomers are combined as follows: 64.0 g Lupersol 70, 240 g ethyl acrylate 360 g glacial acrylic acid. The composition is butyl acrylate/acrylic acid 77.5/22.5 (wt. %) and the weight of Lupersol 70, which is 75% t-butylperacetate, gives 3 wt. % initiator, based on monomer. The monomer solution is added at a constant rate over a 3 hour period to the system maintained at reflux. The solution temperature kept at 152° C.±3° C. Reflux is maintained for 20 minutes after the completion of the addition and then a chaser solution of 10.6 g Lupersol 70 (0.5 wt. % initiator on monomer) in 460.0 g Cellosolve acetate is added over 30 minutes. The system is kept at reflux for 15 minutes following the chaser addition. At this point, solids are 39.6% (solids conditions: 1 g of sample/150° C./1 hr.), indicating essentially complete conversion of monomer.

The pale gold clear solution is cooled and then stripped on a Buchler flask evaporator. Stripping is started at 55° C. and continued until a final temperature of 181° is reached about 4 hours later. The clear, slightly amber product material has a solids content of 98%. The material flows at room temperature. Molecular weights as determined by gel permeation chromatography are M_(w) 2,700 and M_(n) 1,500, and is quite viscous, and the heterogeneity index was 1.8.

The polymers of has a calculated glass transition temperatures of about to 22° C., but the actual Tg is much lower.

EXAMPLE C Preparation of Anionically Initiated EA Oligomer

This is prepared similarly to part 1 of Example A, but is not saponified.

A 900 gram portion of the oligomer at 80% solids in toluene was quenched with 9.5 g of concentrated sulfuric acid. In order to insure salt removal, 2 weight percent, on solids, alum (as a 33% aqueous solution) was charged to the flask and stirred for one hour at 75° C. Water and solvent were then azeotroped out to give a clear yellow solution with large salt crystals. The approximately 100% solids material was filtered to yield a clear yellow fluid having a Tg of -48° C. and a viscosity of 35,500 cps at 25° C. The M_(n) is 1200, the M_(w) is 2400 and the ratio M_(w) /M_(n) is 2.0.

EXAMPLE D Free Radical Oligomer of BA/MMA/AA, 78/15/7

The procedure was essentially the same as Example B.

Instead of mixing the t-butyl peracetate initiator with the monomers and solvent, it is introduced as a separate stream in this example. The reaction was carried out in a four necked 3 liter flask fitted with a stirrer, 500 ml addition funnel (for monomer) two in-line reflux condensers, and a Y joint with thermometer and 250 ml addition funnel (for catalyst solution). The charges were as follows:

    ______________________________________                                         Kettle          725 g  of Cellosolve ® acetate                             Monomers        624 g  butyl acrylate                                                          120 g  methyl methacrylate                                                     56 g   acrylic acid                                            Initiator solution                                                                             275 g  Cellosolve acetate                                                      32 g   Lupersol 70 (t-butyl                                                           peracetate)                                             ______________________________________                                    

The solvent in the kettle is brought to reflux under a nitrogen sparge and the solution temperature is 155° C. The separate streams of catalyst and monomer were commenced and introduced at substantially constant rates over a period of about 21/2 hours.

After vacuum stripping the product at about 20 mm of mercury over a period of about 41/2 hours, the product had a solids content of 98.5%. As elsewhere herein when "solids" is referred to the non-volatile content is meant whether or not the product is a solid or a liquid. The M_(n) is 1150 and the M_(w) is 850. The M_(n) /M_(w) =2.5.

EXAMPLE E Preparation of Free Radical Initiated Oligomer of BA/AA in the Ratio 85/15

This was conducted similarly to Example B and D. The materials used were:

A. 1,000 g Cellosolve acetate

B. 680 g Butyl acrylate

C. 120 g Acrylic acid

D. 32 g Lupersol 70

Component A was charged to a three liter four necked flask with a nitrogen sweep and heated to 150° C. with an oil bath. The addition of premixed components B, C and D solution was commenced at a uniform rate while the temperature was maintained at between 146° C. and 153° C. The monomer addition was completed after 2 hours and 40 minutes and the reactor contents were held at about 150° C. for 15 minutes. The material was stripped at about 20 mm Hg, increasing the solids content to about 95% solids after a final strip at 5 mm Hg at 150°-165° C.

COMPARATIVE EXAMPLE F Emulsion Polymerization of 35EA/55BA/9MAA/1IA with Chain Transfer Agent Introduced into Initial Monomer Emulsion Feed

A monomer emulsion is prepared of 270 grams of water, 350 g of ethyl acrylate, 550 g of butyl acrylate and 90 g of methacrylic acid, 10 g of itaconic acid, and 5 g of 3-mercaptylpropionic acid (0.5% monomers) utilizing an anionic emulsifier, using 1.4 g of ammonium bicarbonate, 50 g of ammonium persulfate. The ammonium bicarbonate and 38 grams of the initiator were introduced into the reactor followed by gradual addition of the remainder of the ammonium persulfate and the monomer emulsion at a relatively constant rate over a period of about 21/2 hours. Residual monomer was chased with tertiary butyl hydroperoxide and Formopon and the product was neutralized with ammonia to a pH of about 5.9

Total solids was 56.1%. The M_(n) is 25,500, the M_(w) is 171,000, and the M_(w) /M_(n) is 6.7.

COMPARATIVE EXAMPLE G Low Molecular Weight Emulsion Polymer of EA/BA/MAA/IA 35/55/9/1 with Chain Transfer Agent added to Emulsion before Polymerization

This example is conducted similarly to Example F utilizing water, 270 g; ethyl acrylate, 350 g; butyl acrylate, 550 g; methacrylic acid, 90 g; itaconic acid, 10 g; 3-mercaptopropionic acid 10 g. The polymer was neutralized to a pH of 6.3 and had a total solids of 54.4%. The M_(n) is 1.4×10⁴, the M_(w) is 0.48×10⁵, and the M_(w) /M_(n) =3.5. Calculated T_(g) is -30° C.

COMPARATIVE EXAMPLE H High Molecular Weight Polymer of EA/IA 99/1 Made with Chaser

The product in this example is 99% ethyl acrylate and 1% itaconic acid. It is made with thermal free radical initiation using an anionic emulsifier and 0.4% ammonium persulfate based on monomers. As is conventional, a chaser catalyst is added after the completion of the addition of monomer emulsion.

This product has a high heterogeneity index of about 67 made without a chain transfer agent but made with a chaser. As has been indicated elsewhere herein heterogeneity index by itself does not give the complete story as to the quantity of high and low molecular weight materials. It gives an indication qualitatively of the molecular weight distribution but does not do so quantitatively. When a chaser catalyst, i.e., an initiator, is added after the all of the monmers are introduced and are nearly fully polymerized (in order to polymerize residual monomer in the aqueous phase and diffused into the polymer particles), such a procedure produces a very small quantity of low molecular weight polymer at the end of the polymerization reaction. As is well known, a large quantity of free radical initiator relative to remaining monomer quantity produces quite low molecular weight materials, especially in emulsion polymerizations. Heterogeneity index thus gives a qualitative indication of the molecular weight distribution but does not do so quantitatively.

To make the monomer emulsion, 9000 g of ethyl acrylate are added to a solution of 44 g of Siponate™ DS-4 (23% sodium dodecylbenzene sulfonate) and 100 g itaconic acid in 2700 g deionized water, and the mixture emulsified (Mixture A). 3588 g of deionized water are placed in a 22 liter kettle fitted with a reflux condenser, thermometer, and facilities for agitation. Agitation is begun and the contents of the kettle heated to 82°-84° C. A solution of 14 g of ammonium bicarbonate dissolved in 300 g deionized water is added to the kettle followed by a solution of 30 g of ammonium persulfate dissolved in 300 g deionized water. Simultaneous addition of Mixture A and a solution of 10 g of ammonium persulfate dissolved in 262 g deionized water is begun and the temperature allowed to rise to 82°-84° C. where it is maintained for the duration of the polymerization. The rates of the additions are adjusted such that additions are complete in 150 minutes. After the additions are complete, a chaser is added in the form of a free radical initiator. Thereafter, the kettle temperature is lowered to 40° C. and a neutralizing solution consisting of a solution of 37.5 g of aqueous ammonia (28% NH₃) in 77.5 g deionized water is added over a period of 15 minutes.

A post additive is next added consisting of a mixture of 200 g of Gelvatol 20-30 (polyvinyl alcohol) and 200 g of Aerotex M-3 (melamine formaldehyde resin) dissolved in 800 g of deionized water. The emulsion is cooled to 25° C. and filtered through a 100 mesh screen to give a product having a pH of 6.8, a Brookfield viscosity of 82 cps and a total solids of 54.3% by weight. The M_(n) is 3×10⁴, the M_(w) is 2×10⁶, and M_(w) /M_(n) is 67.

COMPARATIVE EXAMPLE I High Molecular Weight Polymer Of EA/BA/MAA/IA 35/55/9/1

This product is prepared by the same method as Example H, is neutralized to a pH of 6.2 has a viscosity of 466 cps. at 25° C., using a Brookfield Model LVF Viscomenter Spindle No. 2, at 30 rpm., and a total solids content of 53.1%. The M_(w) ×10⁻⁵ is 8.1 the M_(n) ×10⁻⁴ is 2.0 and the heterogeneity index is 40.5. After the product is cooled to 40° Gelvatol 20-30 and Aerotex M-3 in the same amounts as in Example H are added. A similar procedure was used to prepare a product designated as Example V, but using EA/MMA/IA in the ratios of 87.7/9.5/2.5.

EXAMPLE J-I EA/BA/MAA/IA in the Ratios 35/55/9/1 Using Programed Addition of Chain Transfer Agent to Yield a Mixture of Molecular Weights

In this example the chain transfer agent (CTA), 3-mercaptopropionic acid (MPA), is introduced in two stages. The first stage intercepts the monomer addition at 50% of such addition, and 0.5% of the chain transfer agent, based on the weight of 25% of total monomers, is introduced at that point. At the 75% intercept, 1% of the chain transfer agent based on the weight of the remaining monomers, is introduced. The emulsifier is an anionic emulsifier in the form of Siponate DS-4 which is sodium dodecyl benzene sulfonate. The polymerization is a conventional emulsion polymerization using gradual monomer addition and ammonium persulfate catalyst.

Twenty grams (20 g.) of itaconic acid and 8.8 g. of Siponate DS-4 are dissolved in 540 g. of deionized water. To this is added 700 g. of ethyl acrylate, 1100 g. of butyl acrylate, and 180 g. of methacrylic acid and the mixture emulsified (Mixture A). A cofeed initiator solution is prepared consisting of 2 g. of ammonium persulfate dissolved in 90 g. of deionized water (Solution B). Four hundred eleven grams of deionized water are added to a 5000 ml. four-necked flask fitted with a reflux condenser, thermometer and facilities for agitation and heated to 83° C. Two and eight-tenths grams of ammonium bicarbonate dissolved in 60 g. of deionized water are added to the flask followed by a solution of 6 g. of ammonium persulfate in 60 g. of deionized water. The gradual addition of Mixture A is begun along with the simultaneous addition of Solution B. Polymerization temperature is maintained at 82°-84° C. After 1274 g. of Mixture A and 46 g. of Solution B have been added to the reaction flask over a 75 minute period, the additions are discontinued. After a 5 minute hold period at 82°-84° C., 5 g. of 3-mercaptopropionic acid are added to the remaining Mixture A and the simultaneous gradual addition of Mixture A and Solution B to the reaction flask is resumed. After 640 g. of Mixture A and 23 g. of Solution B have been added to the reaction flask over a 38 minute period, the additions are discontinued. After a 5 minute hold period at 82°-84° C., 2.5 g. of 3-mercaptopropionic acid are added to the remaining Mixture A and the simultaneous gradual addition of Mixture A and Solution B to the reaction flask is resumed and completed in 38 minutes. Thirty minutes after the additions are complete, the temprature is lowered to 40° C. and a solution of 14 g. of ammonium bicarbonate and 12 g. of aqueous ammonia (28% NH₃) in 234 g. of deionized water is added over a period of 20 minutes. The M_(n) is 2×10⁴, M_(w) is 8.1×10⁵, and the M_(w) /M_(n) is 39.6. Fifteen minutes after the addition of this neutralizer, a mixture consisting of 40 g. of Aerotex M-3 (melamine formaldehyde resin) and 100 g. of Gelvatol 20-30 (polyvinyl alcohol) dissolved in 400 g. of deionized water is added. After 15 minutes, the emulsion is cooled and filtered to give a product with a pH of 6.4 and a solids of 52% by weight.

Products were prepared in the same way using the monomer ratios EA/BA/MAA/IA of 34.75/55/9/1.25, 35.5/55/9/0.5, and 41/55/2/2 which had similar properties.

EXAMPLE J-2 Programmed CTA Addition at Different Intercepts, than Example J-1 of Monomer Feed, EA/BA/MAA/IA, 35/55/9/1

This was conducted the same way as Example J-1, but the first addition is 0.33% CTA when 60% of the monomers have been introduced, and the second CTA addition is after 85% of the monomers have been fed, and is 1.0% based on the remaining 15% of monomers.

Itaconic acid, 21 g, and 9.3 g of Siponate DS-4 (sodium dodecyl benzene solfonate) are dissolved in 567 g of deionized water. To this is added 735 g of ethyl acrylate, 1155 g of butyl acrylate, and 189 g of methacrylic acid and the mixture emulsified (Mixture A). 563 g of deionized water are added to a 5000 ml four-necked flask fitted with a reflux condenser, thermometer and facilities for agitation and heated to 83° C. 6.3 g of ammonium persulfate are dissolved in 45 g of deionized water and added to the flask followed by 13 g of a preform consisting of a 45% solids acrylic emulsion polymer having a particle size of 0.09-0.1 microns and a composition consisting of butyl acrylate--52 parts, methyl methacrylate--46.7 parts and methacrylic acid--1.3 parts by weight. The gradual addition of Mixture A is begun along with the simultaneous addition of a cofeed initiator solution consisting of a 2 weight percent solution of ammonium persulfate in deionized water (Solution B). Polymerization temperature is maintained at 82°-84° C. After 1606 g of Mixture A and 64 g of Solution B have been added to the reaction flask over a 90 minute period, the additions are discontinued. After a 10 minute hold period at 82°-84° C., simultaneous gradual addition of Mixture A and Solution B to the reaction flask is resumed along with the simultaneous addition of a solution of 1.78 g 3-mercaptopropionic acid in 32.6 g of deionized water (Solution C). After 669 g of Mixture A, 27 g of Solution B and Solution C have been added to the reaction flask over a period of 40 minutes, the additions are discontinued. After a 10 minute hold period at 82°-84° C., the remaining Mixture A (401 g) and 16 g of Solution B are simultaneously added to the reaction flask along with the simultaneous addition of a solution of 3.11 g of 3-mercaptopropionic acid in 16.4 g of deionized water (Solution D) over a period of 20 minutes. After the additions are complete, the temperature is lowered to 40° C. and a solution of 23 g of aqueous ammonia (28% NH₃) in 202 g of deionized water is added over a period of 20 minutes. The calculated T_(g) is -30° C. This product gave excellent results. The M_(w) /M_(n) was, at the high end of the range of 15-150, but could not be precisely determined because a fraction thereof was insoluble in the usual solvents used in gel permeation chromatography.

After the addition of the neutralizer, a mixture consisting of 42 g of Aerotex M-3 (melamine formaldehyde resin) and 126 g of Gelvatol 20-30 (polyvinyl alcohol) dissolved in 504 g deionized water is added.

EXAMPLE K Preparation of Polymer with High Molecular Weight First Stage and Low Molecular Weight Second Stage of Different Monomer Composition; EA/BA/MAA/IA//BA/AA in the Ratios 31.5/49.5/8.1/0.9//8.5/1.5

The first stage emulsion was water, 292 g; EA, 378 g; BA, 594 g; MAA, 97.2 g; IA, 20.8 g. The second stage was water, 32 g; BA, 102 g; AA, 18 g; 3-MPA, 3.6 g (chain transfer agent).

The polymerization, using Siponate DS-4 as the emulsifier, was conducted similarly to the other emulsion polymerizations above but the first emulsion was introduced over a period of about 2.5 hours, after which the second monomer emulsion feed was commenced, and completed over a period of 35 minutes. The product was cooled to 40° C., and neutralized to a pH of 6.3. It had a viscosity of 6.6 cps with spindle no. 1, 60 rpm using a Brookfield LVF viscometer. The product had a total solids content of 53.6%, and had properties similar to those of Exampls J-1 and J-2.

EXAMPLE L High Molecular Weight Latex Polymer of BA/IDMA/IA in the Ratios 49.5/49.5/1

When these monomers are used similarly to Example H, comparable results were obtained.

Similar results were obtained with BA/IDA/IA in the ratios 49.5/49.5/1 and BA/LMA/IA in the ratios 49.5/49.5/1

EXAMPLE M Low Molecular Weight Emulsion Polymer of EA/BA/MAA/IA, 35/55/9/1

This example was conducted similarly to Example G but utilized 0.1% mercaptopropionic acid in the emulsion feed. The M_(w) ×10⁻⁵ is 17.2, the M_(n) ×10⁻⁴ is 3.2, and the M_(w) /M_(n) equals 53. The product, tested as described herein, had a lap shear adhesion of 177 lb., a 158° F. static cleavage of 1.5 hr., with the early grab strength of fair to good, legs poor and fusion, poor.

COMPARATIVE EXAMPLE N Low Heterogeneity Index Latex Polymer of EA/BA/MAA/IA, 35/55/9/1 0.3% CTA

This is conducted similarly to Example G but with 0.3% mercaptopropionic acid resulting in an M_(w) ×10⁻⁵ of 3.5, and an M_(n) ×10⁻⁴ of 4.0, giving a heterogeneity index, M_(w) /M_(n), of 8.8.

This demonstrates that when a larger amount of chain transfer agent is utilized it is not consumed as rapidly and reults in a more uniform and narrow distribution of molecular weights.

COMPARATIVE EXAMPLE O Low Heterogeneity Index Latex Polymer EA/BA/MAA/IA 35/55/9/1, 0.5% CTA

This was conducted similarly to Example G and the foregoing example, utilizing 0.5% mercaptopropionic acid, resulting in an M_(w) ×10⁻⁵ of 1.26 and M_(n) ×10⁻⁴ of 2.8 and a heterogeneity index of 4.5.

EXAMPLE P Latex Polymer With Staged CTA Introduction EA/BA/MAA/IA, 35/55/9/1

This example was conducted with the same monomers and procedures of Example K but the initial polymerization to achieve a high molecular weight polymer fraction was achieved by feeding 85% of the monomer emulsion before introduction of the chain regulator or chain transfer agent, which was in the amount of 3% based on the remaining monomer, and was 3-mercaptopropionic acid, with only one stage in which the chain transfer agent was used. The M_(w) ×10⁻⁵ is 6.8 the M_(n) ×10⁻⁴ is 1.4 and the M_(w) /M_(n) was 47.

EXAMPLES Q, R, S AND T

These were conducted similarly to Example J-1

COMPARATIVE EXAMPLE V High Molecular Weight Polymer of EA/MMA/IA 88/10/2

This was made similarly to Example H.

Additional examples of polymers prepared similarly to Example H, using no chain transfer agent but using a chaser catalyst, include the following. In cases where there is a double slant line (//) between monomer increments, the meaning is that a first emulsion polymer was prepared of the composition indicated before the double slant line and a second monomer feed was then polymerized with the monomers following the double slant line.

    ______________________________________                                         COMPOSITION         PROPORTIONS                                                ______________________________________                                         BA/VV10/IA          49.5/49.5/1                                                BA/VV10/IA          49.5/50/0.5                                                BA/VV10/IA/BDA      47.8/50/2/0.2                                              BA/VV10/IA/BDA      63.5/35/1.25/0.1                                           BA/VV10/IA          78/20/2                                                    BA/VV10/IA/BDA      77.8/20/2/0.2                                              BA/VV10/IA/BDA      63.65/35/1.25/0.1                                          VA/VV10/IA          48/50/2                                                    VA/VV10/IA          79.5/20/0.5                                                VA/VV10/IA/BDA      79.3/20/0.5/0.2                                            BA/MMA/IA           49.5/49.5/1                                                BA/IDA/IA           41.5/49.5/1                                                BA/IDMA/IA          49.5/49.5/1                                                EHA/MMA/IA          90/9/1                                                     EHA/MAA/IA          94/5/1                                                     EA/BA/MAA/IA        8/82/9/1                                                   EA/BA/MAA/IA        44/55/9/2                                                  EA/BA/MAA/IA        34.5/55/9/1.5                                              ______________________________________                                    

These are suitable as high molecular weight (predominantly) components of blends with low molecular weight polymers. Of course, similar monomer compositions are useful with programmed CTA addition.

The results are summarized in Table I, which gives typical examples within the preferred embodiments of the invention, and comparative examples I.

Table II shows the results of contact adhesives, prepared by utilizing, as in Example H, the same relative quantities of Gelvatol 20-30 and Aerotex M-3.

                                      TABLE I                                      __________________________________________________________________________     Molecular Weight Modifications of Monomer Ratios of Ex. I                      EA/BA/MAA/IA - 35/55/9/1                                                       Type of Molecular Weight Control                                                                   Ex M                                                                               Ex N                                                                               Ex F                                                                              Ex G                                                                               Ex P                                                                              Ex Q                                                                              Ex R                                                                              Ex J-1                                                                             Ex S                                                                              Ex T                        __________________________________________________________________________     Homogeneous addition of chain regulator                                        % Chain             0.1 0.3 0.5                                                                               1.0 -- -- -- --  -- --                          Regulator                                                                      Stepwise Addition of Chain Regulator                                           Intercept No. 1                                                                % of monomer        --             85 25 33 50  50 50                          polymerized                                                                    % chain regulator on remaining                                                                     --             3  0.5                                                                               0.5                                                                               0.5 0.25                                                                              0.5                         monomer                                                                        Intercept No. 2                                                                % of monomer        --             -- 50 66 75  75 75                          polymerized                                                                    % chain regulator on remaining                                                                     --             -- 1.0                                                                               1.0                                                                               1.0 0.5                                                                               0.75                        polymer                                                                        Molecular Weight                                                               --M.sub.w × 10.sup.-5                                                                        17.2                                                                               3.5 1.26                                                                              0.48                                                                               6.8                                                                               7.6                                                                               11.2                                                                              8.1 7.4                                                                               7.2                         --M.sub.n × 10.sup.-4                                                                        3.2 4.0 2.8                                                                               1.4 1.4                                                                               2.0                                                                               2.3                                                                               2.0 3.5                                                                               2.1                         --M.sub.w /M.sub.n  53  8.8 4.5                                                                               3.5 47 37 49 40  21 34                          __________________________________________________________________________

                                      TABLE II                                     __________________________________________________________________________     Comparative Performance of Various Molecular Weight Blends                                                     Ex. I Molecular Weight Modifications                           Ex. U                    Physical                                              Neoprene                 Blend.sup.1                                           Latex*                                                                               Ex. V                                                                               Ex. I                                                                               Ex F                                                                               Ex N Ex. W                                                                               Ex J-1                                                                              Ex. J-2                     __________________________________________________________________________     Intercept No. 1                                                                % of monomer    --    --   --   0   85   --   50   60                          polymerized                                                                    % chain regulator on remaining                                                                 --    --   --   0.5 3    --   0.5  0.33                        monomer                                                                        Intercept No. 2                                                                % of monomer    --    --   --   --  --   --   75   85                          polymerized                                                                    % chain regulator on remaining                                                                 --    --   --   --  --   --   1.0  1.00                        monomer                                                                        Lap Shear       180-200                                                                              180-200                                                                             200-230                                                                             17  170-210                                                                             190-200                                                                             150-160                                                                             271                         Adhesion (psi)                                                                 158° F. cleavage                                                                        2-6   6    6    0.1 6    1.5-2.5                                                                             0.3-1.5                                                                             6                           Resistance (hrs)                                                               Combinability                                                                  Strength        F-G   F-VG P-F  F-G G-Exc.                                                                              F-VG G-VG VG-Exc                      Legginess       VG-Exc                                                                               Poor O-P  G-VG                                                                               G-VG G-VG F-G  VG-Exc                      % Fusion        Exc   Poor O-P  G-VG                                                                               FB.sup.2                                                                            10-FB.sup.2                                                                         20-FB.sup.                                                                          FB.sup.2                    Cycles          none  5    none --  --   --   --   5                           Freeze/Thaw                                                                    __________________________________________________________________________      .sup.1 Blend of Ex I  35 parts/Ex M  22 parts/Ex N  22 parts/Ex O  22          parts;                                                                         .sup.2 Broke Formica                                                           *Fastbond ™ 30                                                        

Table III demonstrates blends of oligomers with the high molecular weight polymer of Example I, as compared with the polymer of Example I, and with a commercial formulated neoprene latex.

Table IV gives comparisons of Example J-2 of the invention involving programmed chain terminator, Example V, a high molecular weight latex polymer made with a chaser but without chain terminator, a commercial acrylic latex product of Union Carbide, and a commercial neoprene latex, Fastbond 30, of 3M Company.

                                      TABLE III                                    __________________________________________________________________________     Aqueous Contact Adhesives From High Molecular Weight Polymer of Ex. I//        Oligomer Blends                                                                Oligomer of                                                                    Example               A.sup.1   B.sup.3                                                                              C.sup.2                                                                              D.sup.4                                                                               E.sup.5 I.sup.6             __________________________________________________________________________     Approx. solids        78% in    100%  100%  100%   100%    --                                        MBK/Tol                                                  Approx. --M.sub.n     1,300     2,000 1,000 2,500  2,000   --                  Conc. of Oligomer in Adhesive                                                                        --    25% 10%                                                                               25%                                                                               10%                                                                               25%                                                                               10%                                                                               25% 10% 25% --                  (solids/basis)                                                                 Early Grab Ratings Very Light Finger Press.                                    Strength              O     F-G O  P-F                                                                               O  O  O  O   O   O   O                   Legs                  O     F-G O  P  O  O  O  O   O   O   O                   Fusion                O     F-G O  O-P                                                                               O  O  O  O   O   O   O                   4.5 lb. Roller Press.                                                          Strength              P-G   VG  G  G  F  P  O  G   G-VG                                                                               F-G O-F                 Legs                  F     G-VG                                                                               F  G  P  P  O  G-VG                                                                               G-VG                                                                               P-F O                   Fusion                F     G-VG                                                                               P  P-F                                                                               O-P                                                                               P  O  F-G G   P   AF.sup.7            Lap Shear Adhesion (lbs.)                                                                            197   104 221                                                                               142                                                                               120                                                                               131                                                                               172                                                                               112 204 155 236                 158° F. Cleavage Resistance (hours)                                                           0.5   0.4 0.4                                                                               0.2                                                                               6  5.3                                                                               1.3                                                                               3.7 2.3 0.9 6                   __________________________________________________________________________      .sup.1,2 Anionically initiated oligomer                                        .sup.3-5 Free radical initiated oligomer                                       .sup.6 Free radical initiated high --M.sub.w polymer                     

                                      TABLE IV                                     __________________________________________________________________________     Comparative Performance of Various Systems                                                            Union Carbide.sup.2                                                            Ucar 154/BKUA-                                                        Ex. J-2.sup.1                                                                       Ex. V.sup.1                                                                        2370, 100/63                                                                            Fastbond-30.sup.3                              __________________________________________________________________________     Lap Shear Adhesion                                                                           200  180 148      175                                            (psi)                                                                          158° F. Static Cleavage                                                               3    5   0.5      1.5                                            (hrs.)                                                                         Resistance to Edge Lift                                                        140° F.                                                                               Pass Pass                                                                               Fail     Pass                                           158° F.                                                                               Pass Pass                                                                               Fail     Pass                                           Early Grab (light pressure)                                                    Strength      Exc  Fair                                                                               Exc      Fair                                           Legs          VG   Poor                                                                               Fair     VG-Exc                                         Fusion        Formica                                                                             5%  Formica  90%                                                          broke    broke                                                   Formica Test Initial grab                                                                    Good None                                                                               Good     Good                                           (no pressure)                                                                  Lamination Pressure                                                                          Light                                                                               Heavy                                                                              Light    Light                                          Required                                                                       Degree of Fusion                                                                             40%  30% 80%      95%                                            Bond Strength VG   Fair                                                                               VG-Exc   Good                                           Type Failure  C/WF CF  C/WF     AF                                             __________________________________________________________________________      .sup.1 Tested as neat emulsions  Not formulated with thickener and             crosslinker.                                                                   .sup.2 A completely formulated system. Ucar 154 unformulated has poor          combineability.                                                                .sup.3 A formulated neoprene latex commercial product.                         CF = Cohesive Failure                                                          C/WF = Cohesive and wood failure (mixed).                                      AF = Adhesive Failure                                                     

We claim:
 1. In an aqueous contact adhesive which dries to an essentially non-tacky state and adheres only to another dried, non-tacky layer of the adhesive upon contact, in the form of an aqueous latex of an addition polymer or polymer mixture at least predominantly of α,β-monoethylenically unsaturated monomers and containing a thickener and crosslinking means, the improvement in which the molecular weight of the polymer or polymer mixture is widely divergent and has a heterogeneity index, the ratio of M_(w) /M_(n) of from 15 to 150, a fraction of from about 5% to 70% of the polymer having an M_(n) of between about 500 and 100,000 and a fraction of from about 30% to 95% of the polymer having an M_(n) of between about 100,000 to at least 2,000,000, the molecular weights being determined by gel permeation chromatography, and the T_(g) of the polymer or polymer mixture is between 10° C. and -60° C. said contact adhesive, when dried to an essentially non-tacky state, adheres only to another dried, non-tacky layer of the adhesive upon contact.
 2. The adhesive of claim 1 in which from about 10% to 60% of the polymer has an M_(n) of from about 10,000 to 50,000, from about 40% to 90% of the polymer has an M_(n) of from about 500,000 to 1,500,000, optionally with fractions of intermediate molecular weights, the heterogeneity index is between 20 and 90, from 12% to 125% of an inorganic filler and a crosslinker are present.
 3. The composition of claim 1 prepared by blending (a) an emulsion of a low molecular weight acrylic polymer obtained by bulk or solution polymerization or a low molecular weight emulsion polymer with (b) a high molecular weight emulsion polymer.
 4. The adhesive of claim 1 prepared by emulsion polymerizing an acrylic emulsion in stages, in which during at least one stage a high molecular weight polymer is obtained, and in which at least one other stage a low molecular weight fraction is obtained.
 5. The composition of claim 4 prepared in stages in a reactor in which at least one portion of the polymerization reaction is conducted in the absence of a free chain transfer agent and at least one stage of the reaction is conducted in the presence of a free chain transfer agent, said T_(g) being from about -5° C. to about -40° C.
 6. The composition of claim 5 in which a first portion of the polymerization is conducted in the absence of a chain transfer agent, or only a low level thereof, whereby a high molecular weight fraction of the polymer is obtained and at least one additional stage of the polymerization is conducted in the presence of a higher amount of a chain transfer agent, whereby at least one low molecular weight fraction is obtained.
 7. The composition of claim 6 in which at least about 30% of the monomers are polymerized in said first stage in the absence of chain transfer agent.
 8. The composition of claim 7 in which there are at least two stages in which monomer and chain transfer agent are introduced, the quantity of chain transfer agent being greater in the last stage of the reaction, there thus being a first polymerization stage in the absence of chain transfer agent, a second polymerization stage in which a chain transfer agent is utilized, and a third stage in which a larger amount of chain transfer agent is utilized.
 9. The composition of claim 8 in which 30% to 90% of the monomer mix has been reacted when the initial chain transfer agent is introduced, and a second introduction of chain transfer agent takes place when between about 50% and 95% of the monomer mix has been reacted.
 10. The composition of claim 9 in which the initial introduction of chain transfer agent takes place when at least 45% of the monomers have been introduced into the reactor and polymerized, and the second introduction of monomer and chain transfer agent takes place when at least 60% of the monomers have been introduced into the reactor and polymerized, the thickener is polyvinyl alcohol, and the crosslinking means is a water soluble aminoplast.
 11. The composition of claim 10 in which the polymer contains 0.5-15% of a carboxylic acid selected from the group consisting of acrylic acid, itaconic acid, fumaric acid, methacrylic acid, and crotonic acid, and mixtures thereof, 0-25% by weight of another addition polymerizable α,β-monoethylenically unsaturated monomer, optionally including one or more of an ester of methacrylic acid with an alcohol having from 1 to 18 carbon atoms, vinyl versatate, styrene, vinyl toluene, vinyl acetate and acrylonitrile, the balance of the copolymerized monomers being one or more esters of acrylic acid with an alcohol having from 1 to 18 carbon atoms.
 12. The composition of claim 11 in which the carboxylic acid is a mixture of 5-10% of methacrylic acid with 0.5-2% of itaconic acid, and said ester or esters of acrylic acid have 1 to 8 carbon atoms.
 13. The composition of claim 12 in which the monomers consist essentially of ethyl acrylate, butyl acrylate and said acids.
 14. The composition of claim 13 in which the chain transfer agent is selected from long chain alkyl mercaptans, dialkyl xanthogen disulfides, mercaptocarboxylic acids, alkarylmercaptans, long chain alcohols, and halogenated hydrocarbons.
 15. A method of adhering two surfaces together comprising the steps of applying the composition of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or 14 to the two surfaces to be joined, curing the two coatings to a non-tacky state but a state in which the surfaces adhere to one another upon contact, and mating the surfaces with pressure, thereby bonding the two substrates to one another.
 16. A laminate prepared by the method of claim
 15. 